Conference Agenda

This research and collaboration project aims at the calibration and validation (Cal/Val) of the European Sentinel-3, Sentinel-6 and the Chinese HY-2 satellite altimeters using two permanent Cal/Val facilities: (1) the Permanent Facility for Altimetry Calibration established by ESA in Crete, Greece and (2) the National Altimetry Calibration Cooperation Plan of China. Other satellites, such as the Guanlan, CryoSat-2, CFOSAT, CRISTAL, etc., may also be supported by these Cal/Val infrastructures.

Satellites will be calibrated and monitored using uniform, standardized procedures and protocols while exploiting trusted and indisputable reference standards at both Cal/Val infrastructures in Europe and China. At present, the PFAC, Greece implements the action plan established by ESA for Fiducial Reference Measurements for Altimetry and reports its Cal/Val results along with their FRM uncertainty.

Through the ESA Dragon-5 project, the FRM procedures, protocols and best practices, will be updated, upgraded and followed at both Cal/Val facilities in Europe and China. Calibration of altimeters is accomplished by examining satellite observations in open seas against reference measurements. Comparisons are established through precise satellite positioning, water level observations, GPS buoys and reference models (geoid, mean dynamic topography, earth tides, troposphere and ionosphere) all defined by Cal/Val sites. The final uncertainty (FRM status) for altimeter bias will be attributed to several individual error sources, coming from observations in water level, atmosphere, absolute positioning, reference surface models, transfer of heights from Cal/Val sites to satellite observations, etc.

During this first year, the following tasks are been carried out:

• The European CRS1 Cal/Val site in the west coast of Crete, Greece and the Wanshan, Zhuhai, China Cal/Val facility have been employed for Sentinel-3A, Sentinel-3B and HY-2B satellite altimeters calibration and validation;
• The CRS1 Cal/Val site is equipped with two tide gauges (radar and pressure), one GNSS station and one weather stations. A second European Cal/Val site currently operating for Sentinel-3A and Sentinel-6MF mission will be also employed for the calibration of HY-2B/C satellite altimeters;
• The Wanshan Cal/Val facility comprises from four sites at Wailingding, Dangan, Zhiwan and Miaowan islands. These sites are equipped with four GNSS receivers, three acoustic tide gauges, one automatic weather station and one solar photometer. A moored meteorological buoy and a GNSS buoy will be installed in May 2021 25 km south of Zhiwan island;
• Determination of the FRM uncertainty in both (European and Chinese) Cal/Val sites;
• Relative calibration of the HY-2B altimeter with respect to Sentinel-3 via crossover analysis.
• Inter-comparison of the Cal/Val results obtained at the two Cal/Val facilities and investigation of any deviations.

The main findings of the joint work carried out by the European and Chinese teams so far are:

• Both European and Chinese Cal/Val sites are operational and implement independent calibration methodologies for European and Chinese satellite altimetry missions (i.e., Sentinel-3A/B, HY-2B/C, etc.);
• Standardization of operations and data processing followed by both Cal/Val infrastructures in Europe and China is a complex procedure as different instrumentation/models and processing algorithms/strategies are employed whereas deviations also arise due to the geographic location (i.e., bathymetry, tides, etc.) of each Cal/Val facility;
• Identification of several common procedures identified (i.e., tide gauge quality control, GNSS positioning, etc.) which pave the path towards the standardization of operations and data processing.
• Products of HY-2B satellite altimeter differ depending on the repository used. Quality control and standardization of these satellite measurements shall be secured prior to any Cal/Val activity;
• The performance of the HY-2B and HY-2C satellite altimeter is within the mission requirements.

Respect to the objectives and schedule of our project, the first-year report will include on-going activities and results of Bufeng-1 data processing, calibration workflow, and validation of the calibrated results on hurricane winds, soil moisture, and sea level measurements. The presentation has three parts. Firstly, a short introduction will be given about Bufeng-1 twin satellites that carry the Chinese first generation spaceborne GNSS-R instruments started using reflected GNSS signals to perform earth observation. Secondly, by utilizing the Bufeng-1 Normalized Bistatic Radar Cross Section (NBRCS), earth reflectivity, and range measurements, the preliminary results show that BuFeng-1 has a high agreement compared with other observations on severe sea surface winds, soil moisture, and sea level. In this presentation, the measurements of Bufeng-1 will be aligned with SFMR collected hurricanes, SMAP derived soil moisture, and DTU10 sea level models. Then, the validations of the accuracy and correlation coefficients will be analyzed to discuss the limitations and issues for the future research. For the last part, we will give the outlook about our future works of the objectives and the future plan of Bufeng missions.

This report provides an overview of the recent progress on the cross-calibration and validation of CSES/Swarm satellite magnetic field and plasma measurements. Preliminary validation of the high precision magnetometer (HPM) measurements from CSES has shown good agreement with magnetic field measurements from Swarm. The HPM Level-2 scientific dataset and data description document, including data format, naming convention, and quality flags, have been released and can now be used as a reference by interested users. This dataset has recently been used to derive the CSES global geomagnetic field model (CGGM), one of the candidate models for the most recent version of the International Geomagnetic Reference Field (IGRF-13). Regarding the electron density and temperature measured by the Langmuir probe onboard CSES, comprehensive comparisons have been performed against measurements from both Swarm and the incoherent scatter radar at Millstone Hill, and predictions from the International Reference Ionosphere (IRI) model. The results showed that the CSES electron density measurements generally agree with the other measurements, though they present a relatively lower absolute value. Two kinds of platform interference on the CSES Langmuir probe have been identified: a sudden drop and another sudden rise of floating potential on the nightside and dayside, respectively, both linked to the adjustment of the current system equilibrium of the CSES platform (i.e., when the satellite flies into/out of the sunlight region).

It is very important to establish a longstanding ground-based FTIR and MAXDOAS measurement dataset at Xianghe that can be applied to evaluate the S5P and FY-3H/GAS and TanSat satellite measurements in northern China region. It is still of few ground measurements of FTIR and MAXDOAS measurement data that can be used as the reference data for validation of satellite observations in China. For the common products (NO₂, O₃ and HCHO) , the simultaneous FTIR and MAXDOAS measurements at Xianghe site allow us to understand their differences before combining them together for satellite validation. For other products (CO, CH₄ and CO₂), more focus will be taken on the ground-based FTIR retrievals. The column-averaged dry-air mole fractions of CO₂ (XCO_­2), CH₄ (XCH₄) and CO (XCO) have been measured with a Bruker IFS 125HR Fourier-transform infrared (FTIR) spectrometer at Xianghe since June 2018. The HCHO data derived from the MAX-DOAS spectrometer and the FTIR instrument operating in parallel at Xianghe station (39.75° N, 116.96° E; ~55 km southeast of Beijing) were used to validate TROPOMI HCHO data products. The comparison results appear consistent with validation results obtained at TCCON sites for XCO₂ and XCH₄.